Advanced converter sludge utilization technologies for the recovery of valuable elements: A review

A holistic valorization of treasured waste activated sludge for directional high-valued products recovery: Routes, key technologies and challenges

Global energy shortages and environmental crises underscore the imperative for a circular economy to tackle resource scarcity and waste management. The circular economy model encourages the recovery and reuse of valuable materials, reducing reliance on finite natural resources and lessening the environmental impact of waste disposal. Among urban organic solid wastes, waste activated sludge (WAS) emerges as a potent reservoir of untapped resources (including various inorganic and organic ones) offering significant potential for recovery. This review delves into a comprehensive analysis of directional valorization of WAS to recover high-valued products, including the inorganic matters (i.e. phosphorus, ammonia nitrogen, and heavy metals), organic resources (i.e. extracellular polymers like alginate and protein, volatile fatty acid, methane, hydrogen, and plant growth hormones) and reutilization of WAS residues for the preparation of adsorbent materials - the biochar. Moreover, the main recovery methodologies associated influencing parameters, product application, and attendant challenges for those diverse recovered resources are unveiled. Future research are encouraged to prioritize the development of integrated multi-resource recovery approaches, the establishment of regulatory frameworks to support resource recovery and product utilization, and the systematic evaluation of disposal strategies to foster a more sustainable and resource-efficient future. This work illuminates avenues for sustainable WAS management with high-valued resource recovery towards circular economy.

Characteristics and Sintering Mechanisms of Iron Ores with a High Proportion of High-Al2O3 Limonite

In this study, a sintering test of high-alumina limonite from Indonesia, matched with an appropriate magnetite concentration, is performed. The sintering yield and quality index are effectively improved by optimizing the ore matching and regulating the basicity. For the optimal coke dosage of 5.8% and basicity of 1.8, the tumbling index of the ore blend is found to be 61.5% and the productivity is 1.2 t/(h·m²). The main liquid phase in the sinter is the silico-ferrite of calcium and aluminum (SFCA), followed by a mutual solution, both of which maintain the sintering strength. However, when the basicity is increased from 1.8 to 2.0, the production of SFCA is found to increase gradually, whereas the mutual solution content decreases dramatically. A metallurgical performance test of the optimal sinter sample demonstrates that the sinter can meet the requirements of small- and medium-sized blast furnace smelting, even for high-alumina limonite ratios of 60.0-65.0%, thereby greatly reducing the sintering production costs. The results of this study are expected to provide theoretical guidance for the practical high-proportion sintering of high-alumina limonite.

Remarkable enrichment of heavy metals in baghouse filter dust during direct-fired thermal desorption of contaminated soil

This study focuses on the widely applied technology of direct-fired thermal desorption, taking a site contaminated by polycyclic aromatic hydrocarbons (PAHs) as a typical test case. The entire thermal desorption process of contaminated soil is considered in the analysis. The concentration levels and occurrence characteristics of heavy metals in dust traditionally considered to be clean are evaluated, and possible secondary pollution and environmental impacts are explored. The results indicate that, compared with the thermal desorption soil, the dust samples generated in the baghouse filter during the ex situ direct-fired thermal desorption process have higher amounts of heavy metal accumulation as well as altered speciation. In addition, the enrichment characteristics and origins of the heavy metals are analyzed according to the process flow and particle size composition as well as the results of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron probe microanalysis (EPMA), and other microscopic research methods. Phenomenon further reveals enrichment of arsenic (As), nickel (Ni), and chromium (Cr). The findings of this study can provide a scientific basis for the proper disposal and risk management of the dust collected after direct-fired thermal desorption treatment of contaminated soil.

Co-hydrothermal carbonization of sewage sludge and polyvinyl chloride for the production of high-quality solid fuel with low nitrogen content

Sewage sludge (SS) and polyvinyl chloride (PVC) are typical solid wastes. Their co-hydrothermal carbonization behavior was investigated in this study. The low-nitrogen solid fuel (0.94 wt%) with high heating value (9.84 MJ·Kg^-1) was prepared through parameter optimization at 240 °C for 1.5 h under water loading amount of 0.84 g·cm^-3. In an acidic environment, the stubborn protein in SS could be converted into free amino acids, which were generated by the decomposition of PVC under hydrothermal conditions. The stubborn N could be translated into easy-to-remove N, such as nitrile-N and inorganic N, and the dehydration reaction was evidently promoted. The acidic environment at high temperatures caused the dissolution of ash in SS and improved the combustion performance of hydrochar. FT-IR results showed that, with increased PVC loading proportion, -C=N- was converted into -C=O-. Co-hydrothermal carbonization could effectively improve the combustion performance of hydrochar. The addition of PVC could lead to the generation of increased volatile matter, fixed carbon, and unsaturated CC, and the combustion temperature range shifted to a high range. However, the generation of graphite-like carbon was caused by further increasing the PVC loading proportion, which hindered the improvement of its combustion performance. In the parameter optimization study, the increased water loading amount (from 0.54 g·cm^-3 to 0.84 g·cm^-3) had the most evident effect on the N content in the hydrochar (from 1.50 wt% to 0.94 wt%), which promoted the denitrification efficiency (from 60.11% to 75.00%) and the conversion of -C=N- components, and prevented further polymerization of solid products.

Co-combustion of distillery sludge and coal for application in boiler and subsequent utilization of the generated bottom ash

Combustion stands as one of the essential methods in resource recovery for disposal of distillery sludge. In this study, sludge along with coal has been considered an option for co-combustion in the grate furnace aiming for further application as a boiler fuel. Detailed analysis was carried out to verify the feasibility of co-combustion of sludge with coal. Distillery sludge was blended with coal as a mixed fuel at co-combustion ratios of 20%, 30%, and 40% in grate furnace. The results of the analysis indicated that the combustion with 40% sludge mixed coal is suitable for application as a fuel in boiler. According to the chemical composition of bottom ash, weight loss from 460 to 800°C indicated the presence of C-C and C-H. Also, EDX and XRD analyses of mixed fuel was carried out to determine the mineralogical composition. The presence of quartz (SiO₂), mullite (3Al₂O₃2SiO₂), and hematite (Fe₂O₃) present in the ash can be used as mineral additives in cement industries. The study also provided a promising approach towards diverting combustion bottom ash from landfills for its utilization in various industries which can be a possible cost-effective solution.

Harmless treatment of municipal solid waste incinerator fly ash through shaft furnace

Municipal solid waste incinerator fly ash (MSWI FA) is a type of waste that is harmful to the environment, and the melting treatment methods can treat MSWI FA, removing its potential negative impacts. However, special equipment is required for the FA melting process, which necessitates high costs. Metallurgical shaft furnaces (MSF) can melt MSWI FA efficiently. Therefore, the feasibility of using an MSF for FA treatment was studied herein. First, the fundamental physicochemical properties of the FA were analyzed. Then, the appearance and internal morphology of the FA were examined using a scanning electron microscope. Finally, melting experiments were designed according to the conditions of the MSF. The results show that slag changes into a glassy state under rapid cooling, which is beneficial to the solidification of harmful elements. These harmful elements, including Pb, Zn, and Cu, are thus reduced and volatilized into the flue gas under the MSF's reducing atmosphere. The harmful elements that enter the slag are solidified, causing its leaching toxicity to achieve the national standard requirements. Further, under the simulated MSF smelting conditions, the FA dioxin destroy removal efficiency realized more than 99.99% efficiency. Therefore, the harmless treatment of MAWI FA can be realized through MSF process.

Bioaugmentation coupled with phytoextraction for the treatment of Cd and Sr, and reuse opportunities for phosphogypsum rare earth elements

The environmental and health impacts caused by phosphogypsum (PG) make it necessary to carefully manage these wastes. Bioaugmentation of a PG-compost mix with Bacillus cereus was associated with Trifolium pratense or Helianthus annuus for the phytoextraction of metal trace elements (MTE). In hydroponics, MTE concentrations in sunflower shoots are higher than in clover; however, as opposed to clover, it regulates their accumulation. The MTE accumulation levels by plants cultivated in pots with the PG-compost mix are much lower than in hydroponics due to lower concentration in available MTE. The bacteria-plant coupling has served to raise MTE concentrations, especially for rare earth elements (REE), i.e., Ce, La, Nd, Y, in the AP of sunflower, by factors of 4.4, 38.3, 3.4 and 21, respectively, compared to non-bioaugmented control. The translocation factor was also increased for all MTE and is ranged between 1.1 for Sr and 6.8 for Y. Moreover, the presence of bacteria raises plant biomass by a factor of 3.7 for shoots and 2.9 for the roots as regards clover. Results showed that in addition to phytoextraction of REE elements, all providing the promise of some kind of economic opportunity, the dispersion of PG stockpiles dust and erosion should be reduced.

A new method for removal of nitrogen in sewage sludge-derived hydrochar with hydrotalcite as the catalyst

The high content of nitrogen in hydrochar produced from hydrothermal carbonization (HTC) of sewage sludge (SS) leads to serious NOx pollution when the hydrochar is used as a solid fuel. Mg-Ga layered double hydroxides (LDHs), Mg-Al LDHs and their calcined samples (layered double oxides, LDO) were prepared. The LDHs and LDO all can notably promote the removal of nitrogen element, in which organic-N was transferred to NH₄⁺-N to cause increasing pH value. Mg-Al LDO showed the highest efficiency for the removal of nitrogen among the catalysts. The thermal decomposition of the N-organic matter with acidic sites in catalyst was the key step to release NH₃. The key role of basic sites in Mg-Al LDO was that it can effectively destroy the cell wall and extracellular polymeric substances structure. The lipid-like substance did not participate in the carbonization reaction, but they can be absorbed by the hydrochar. Partial SS floc directly transformed to hydrochar according to "solid-solid" reaction. The reaction pathways of remove nitrogen were proposed.